The Entrainment of Circadian Rhythms in Man

Physiological rhythms, both circadian and longer-term (e.g., the reproductive rhythms of many species), are now well-recognized throughout the plant and animal kingdom. Of recent years there has been a considerable revival of interest in the factors concerned with the initiation and maintenance of 24 hr. rhythms, and a great deal of new work has been carried out in many diverse biological fields. It is apparent from surveys of this work, such as those made by Kleitman [1] and Harker [2], however, that although the total amount of data concerning biological rhythms which has been gained both from field studies and from laboratory experimentation is large, the conclusions which have been drawn from this work are often confusing and provocative. The site and nature of the "physio-logical clock" or clocks which control such mechanisms are still very much in question, as also is the precise role of the environment in...